Structure and interactions in fluids of prolate colloidal ellipsoids: Comparison between experiment, theory, and simulation

Abstract

The microscopic structure of fluids of simple spheres is well known. However, the constituents of most real-life fluids are non-spherical, leading to a coupling between the rotational and translational degrees of freedom. The structure of simple dense fluids of spheroids - ellipsoids of revolution - was only recently determined by direct experimental techniques [A. P. Cohen, E. Janai, E. Mogilko, A. B. Schofield, and E. Sloutskin, Phys. Rev. Lett. 107, 238301 (2011)]. Using confocal microscopy, it was demonstrated that the structure of these simple fluids cannot be described by hard particle models based on the widely used Percus-Yevick approximation. In this paper, we describe a new protocol for determining the shape of the experimental spheroids, which allows us to expand our previous microscopy measurements of these fluids. To avoid the approximations in the theoretical approach, we have also used molecular dynamics simulations to reproduce the experimental radial distribution functions g(r) and estimate the contribution of charge effects to the interactions. Accounting for these charge effects within the Percus-Yevick framework leads to similar agreement with the experiment.

Structural Transition in a Fluid of Spheroids: A Low-Density Vestige of Jamming

“…For higher aspect ratios, this procedure is insufficient and particle stabilization is achieved by introduction of the AOT (dioctyl sodium sulfosuccinate, Sigma-Aldrich 98%) micelles into the suspension. The AOT micelles charge the particles, but also screen long-range Coulomb repulsions [5,31], so that the interactions become quasihard at high AOT concentrations [32]. The coherent physical behavior observed in this work for the AOT-and the PHSA-stabilized particles, at different particle aspect ratios t and volume fractions ϕ, suggests that the chemical details of the stabilizing mechanism do not matter for our current purposes.…”

“…This model has recently been realized in colloidal and granular matter experiments, providing an important insight onto the local bulk structure of fluids [4,5] and jammed packings [6]. While a very good agreement between experiment and theory has been achieved for the fluids [5,7], these studies dealt with only one specific particle aspect ratio, t ¼ 1.6. The dependence of the fluid structure on the aspect ratio of the constituent particles has not been tested, so that the fundamental role played in these fluids by rotational degrees of freedom remains unknown.…”

“…A system of spheroids, ellipsoids of revolution, is arguably the simplest model of matter, where such a coupling exists. This model has recently been realized in colloidal and granular matter experiments, providing an important insight onto the local bulk structure of fluids [4,5] and jammed packings [6]. While a very good agreement between experiment and theory has been achieved for the fluids [5,7], these studies dealt with only one specific particle aspect ratio, t ¼ 1.6.…”

“…This solvent mixture matches the density and the refractive index of the colloids, eliminating particle sedimentation and allowing confocal imaging deep into the bulk of the suspensions. The particle stretching procedure damages the PHSA monolayer [4,5,30]. For t ≲ 1.7, the monolayer was replenished by readsorption of PHSA from a concentrated solution [30].…”

“…We prepare our colloidal ellipsoids by a uniaxial stretching of PMMA (polymethymethacrylate) spheres [27][28][29][30], 2.4 AE 0.04 μm in diameter, as described elsewhere [4,5]. The particles are fluorescently labeled with Nile red for confocal microscopy and sterically stabilized by a poly-12-hydroxystearic acid (PHSA) monolayer [4,5,11]. We suspend the ellipsoids in a mixture of decahydronaphthalene and tetrachloroethylene (mass ratio of 1:1.1), where their rotational and positional degrees of freedom are thermalized by the Brownian motion.…”